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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Conceptual Physics Fundamentals Chapter 7: FLUID MECHANICS 1.

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Presentation on theme: "Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Conceptual Physics Fundamentals Chapter 7: FLUID MECHANICS 1."— Presentation transcript:

1 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Conceptual Physics Fundamentals Chapter 7: FLUID MECHANICS 1

2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley This lecture will help you understand: Density Pressure Pressure in a Liquid Buoyancy in a Liquid Archimedes Principle Pressure in a Gas Atmospheric Pressure Pascals Principle Buoyancy in a Gas Bernoullis Principle 2

3 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Fluid Mechanics Men never do evil so cheerfully and completely as when they do so from religious conviction. Blaise Pascal 3

4 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Density important property of materials (solids, liquids, gases) measure of compactness of how much mass an object occupies lightness or heaviness of materials of the same size 4

5 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Density in equation form: units: grams or kilograms volume: cm 3 or m 3 example: density of mercury is 13.6 g/cm 3, so mercury has 13.6 times as much mass as an equal volume of water (density 1 g/cm 3 ) 5

6 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The volume of 1000 kg of water is _______________. A.1 liter B.slightly less than 1 liter C.more than 1 liter D.None of the above Density CHECK YOUR NEIGHBOR 6

7 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The volume of 1000 kg of water is _______________. A.1 liter B.slightly less than 1 liter C.more than 1 liter D.None of the above Density CHECK YOUR ANSWER 7

8 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Density Weight density in equation form: example: density of salt water is 64 lb/ft 3, more dense than fresh water (density 62.4 lb/ft 3 ) 8

9 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Which of these has the greatest density? A.100 kg of lead B.100 kg of water C.Both are the same D.None of the above Density CHECK YOUR NEIGHBOR 9

10 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Which of these has the greatest density? A.100 kg of lead B.100 kg of water C.Both are the same D.None of the above Explanation: They have the same mass and weight, but any amount of lead is more dense than any amount of water. Density CHECK YOUR ANSWER 10

11 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When water freezes, it expands. This tells us that the density of ice is _______________. A.less than the density of water B.equal to the density of water C.more than the density of water D.None of the above Density CHECK YOUR NEIGHBOR 11

12 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When water freezes, it expands. This tells us that the density of ice is _______________ A.less than the density of water B.equal to the density of water C.more than the density of water D.None of the above Density CHECK YOUR ANSWER 12

13 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Compared with the weight of water, the weight of a liter of ice is _______________. A.more B.less C.the same D.None of the above Density CHECK YOUR NEIGHBOR 13

14 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Compared with the weight of water, the weight of a liter of ice is _______________. A.more B.less C.the same D.None of the above Explanation When a liter of water freezes, its volume increases. To get a liter of ice youd have to shave part of it off to be the same size. So there is less water in a liter of ice. Density CHECK YOUR ANSWER 14

15 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure the force per unit area that one object exerts on another in equation form: depends on area over which force is distributed units: N/m 2, lb/ft 2, or Pa (Pascals) 15

16 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure example: The teacher between nails is unharmed because force is applied over many nails. Combined surface area of the nails results in a tolerable pressure that does not puncture the skin. 16

17 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you stand on one foot instead of two, the force you exert on the floor is __________. A.less B.the same C.more D.None of the above Pressure CHECK YOUR NEIGHBOR 17

18 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you stand on one foot instead of two, the force you exert on the floor is ___________. A.less B.the same C.more D.None of the above Explanation: Distinguish between force and pressure! Pressure CHECK YOUR ANSWER 18

19 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you stand on one foot instead of two, the pressure you exert on the floor is _____________. A.less B.the same C.more D.None of the above Pressure CHECK YOUR NEIGHBOR 19

20 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you stand on one foot instead of two, the pressure you exert on the floor is _____________. A. less B.the same C.more D.None of the above Explanation: Twice as much, in fact! Pressure CHECK YOUR ANSWER 20

21 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Liquid force per unit area that a liquid exerts on an object depth dependent and not volume dependent example: swim twice as deep, then twice as much weight of water above you produces as much pressure on you 21

22 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Liquid acts equally in all directions example: o your ears feel the same amount of pressure under water no matter how you tip your head o bottom of a boat is pushed upward by water pressure o pressure acts upward when pushing a beach ball under water 22

23 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Liquid independent of shape of container whatever the shape of a container, pressure at any particular depth is the same in equation form: 23

24 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Water pressure provided by a water tower is greater if the tower __________. A.is taller B.holds more water C.Both A and B D.None of the above Pressure in a Liquid CHECK YOUR NEIGHBOR 24

25 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Water pressure provided by a water tower is greater if the tower ____________. A.is taller B.holds more water C.Both A and B D.None of the above Explanation: Only depth, not amount of water, contributes to pressure. Pressure in a Liquid CHECK YOUR ANSWER 25

26 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Liquid Effects of water pressure acts perpendicular to surfaces of a container liquid spurts at right angles from a hole in the surface curving downward –The greater the depth, the greater the exiting speed 26

27 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in a Liquid Buoyancy apparent loss of weight of a submerged object amount equals the weight of water displaced 27

28 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in a Liquid Displacement rule: A completely submerged object always displaces a volume of liquid equal to its own volume. example: Place a stone in a container that is brim- full of water, and the amount of water overflow equals the volume of the stone 28

29 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A cook who measures a specific amount of butter by placing it in a measuring cup with water in it is using the ___________. A.principle of buoyancy B.displacement rule C.concept of density D.All of the above Buoyancy in a Liquid CHECK YOUR NEIGHBOR 29

30 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A cook who measures a specific amount of butter by placing it in a measuring cup with water in it is using the __________. A.principle of buoyancy B.displacement rule C.concept of density D.All of the above Buoyancy in a Liquid CHECK YOUR ANSWER 30

31 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in a Liquid Buoyant force net upward force that a fluid exerts on an immersed object = weight of water displaced example: the difference in the upward and downward forces acting on the submerged block is the same at any depth 31

32 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley How many forces act on a submerged body at rest in a fluid? A.Onebuoyancy B.Twobuoyancy and the force due to gravity C.Nonein accord with the equilibrium rule, F = 0 D.None of the above Buoyancy in a Liquid CHECK YOUR NEIGHBOR 32

33 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley How many forces act on a submerged body at rest in a fluid? A.Onebuoyancy B.Twobuoyancy and the force due to gravity C.Nonein accord with the equilibrium rule, F = 0 D.None of the above Buoyancy in a Liquid CHECK YOUR ANSWER 33

34 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in a Liquid Sink or float? sink when weight of submerged object is greater than the buoyant force neither sink nor float when weight of a submerged object is equal to buoyant force object will remain at any level float when weight of submerged object is less than the buoyant force it would have when submergedwhen floating, buoyant force = weight of floating object 34

35 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Archimedes Principle discovered by Greek scientist Archimedes relates buoyancy to displaced liquid states that an immersed body (completely or partially) is buoyed up by a force equal to the weight of the fluid it displaces applies to gases and liquids 35

36 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Archimedes Principle Apparent weight of a submerged object weight out of waterbuoyant force example: If a 3-kg block submerged in water apparently weighs 2 kg, then the buoyant force or weight of water displaced is 1 kg. 36

37 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley On which of these blocks submerged in water is the buoyant force greatest? A.1 kg of lead B.1 kg of aluminum C.1 kg of uranium D.All the same Archimedes Principle CHECK YOUR NEIGHBOR 37

38 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley On which of these blocks submerged in water is the buoyant force greatest? A.1 kg of lead B.1 kg of aluminum C.1 kg of uranium D.all the same Explanation: The largest block is the aluminum one. It displaces more water and therefore experiences the greatest buoyant force. Archimedes Principle CHECK YOUR ANSWER 38

39 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a fish expands its air bladder, the density of the fish ________________. A.decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR NEIGHBOR 39

40 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a fish expands its air bladder, the density of the fish _____________. A.decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR ANSWER 40

41 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a fish makes itself less dense, the buoyant force on it ______________. A.decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR NEIGHBOR 41

42 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a fish makes itself less dense, the buoyant force on it ______________. A. decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR ANSWER 42

43 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a fish decreases the size of its air bladder, the density of the fish ___________. A.decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR NEIGHBOR 43

44 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a fish decreases the size of its air bladder, the density of the fish _____________. A. decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR ANSWER 44

45 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a submarine takes water into its ballast tanks, its density __________. A.decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR NEIGHBOR 45

46 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a submarine takes water into its ballast tanks, its density ______________. A. decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR ANSWER 46

47 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a submerged submarine expels water from its ballast tanks, its density __________. A.decreases B.increases C.remains the same D.None of the above Archimedes Principle CHECK YOUR NEIGHBOR 47

48 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When a submerged submarine expels water from its ballast tanks, its density __________. A. decreases B.increases C.remains the same D.None of the above Explanation: This is how a submerged submarine is able to surface. Archimedes Principle CHECK YOUR ANSWER 48

49 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Archimedes Principle Flotation Principle of flotation o a floating object displaces a weight of fluid equal to its own weight example: A solid iron 1-ton block may displace 1/8 ton of water and sink. The same 1 ton of iron in a bowl shape displaces a greater volume of waterthe greater buoyant force allows it to float. 49

50 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The reason a person finds it easier to float in salt water, compared with fresh water, is that in salt water __________. A.the buoyant force is greater B.a person feels less heavy C.Neither of these Archimedes Principle CHECK YOUR NEIGHBOR 50

51 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The reason a person finds it easier to float in salt water, compared with fresh water, is that in salt water __________. A. the buoyant force is greater B.a person feels less heavy C.Neither of these Explanation: A floating person has the same buoyant force whatever the density of water. A person floats higher because a smaller volume of the denser salt water is displaced. Archimedes Principle CHECK YOUR ANSWER 51

52 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley On a boat ride, the skipper gives you a life preserver filled with lead pellets. When he sees the skeptical look on your face, he says that youll experience a greater buoyant force if you fall overboard than your friends who wear Styrofoam- filled preservers. A.He apparently doesnt know his physics. B.He is correct. Archimedes Principle CHECK YOUR NEIGHBOR 52

53 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley On a boat ride, the skipper gives you a life preserver filled with lead pellets. When he sees the skeptical look on your face, he says that youll experience a greater buoyant force if you fall overboard than your friends who wear Styrofoam- filled preservers. A. He apparently doesnt know his physics. B.He is correct. Explanation: Hes correct, but what he doesnt tell you is youll drown! Your life preserver will submerge and displace more water than those of your friends who float at the surface. Although the buoyant force on you will be greater, the net force downward is greater still! Archimedes Principle CHECK YOUR ANSWER 53

54 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Archimedes Principle Denser fluids will exert a greater buoyant force on a body than less dense fluids of the same volume. example: ship will float higher in salt water (density = 1.03 g/cm 3 ) than in fresh water (density = 1.00 g/cm 3 ) 54

55 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Archimedes Principle applies in air o The more air an object displaces, the greater the buoyant force on it. o If an object displaces its weight, it hovers at a constant altitude, o If an object displaces less air, it descends, 55

56 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley As you sit in class, is there a buoyant force acting on you? A.No, as evidenced by an absence of lift B.Yes, due to displacement of air Archimedes Principle CHECK YOUR NEIGHBOR 56

57 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley As you sit in class, is there a buoyant force acting on you? A.No, as evidenced by an absence of lift B.Yes, due to displacement of air Explanation: There is a buoyant force on you due to air displacement, but much less than your weight. Archimedes Principle CHECK YOUR ANSWER 57

58 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Gas greater distance between molecules in a gas than in a liquid molecules in a gas are free from the cohesive forces Molecular motion of gas exerts pressure against a container. For large volumes of gas, gravitational forces limit size and determine shape. 58

59 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Gas Relationship between pressure and density gas pressure is proportional to density example: o Air pressure and air density inside an inflated tire are greater than the atmospheric pressure and density outside. o twice as many molecules in the same volume air density doubled o for molecules moving at the same speed (same temperature), collisions are doubled pressure doubled 59

60 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Gas Double density of air by doubling the amount of air decreasing the volume to half 60

61 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pressure in a Gas Boyles Law relationship between pressure and volume for ideal gases an ideal gas is one in which intermolecular forces play no role states that pressure volume is a constant for a given mass of confined gas regardless of changes in pressure or volume (with temperature remaining unchanged) pressure volume = constant means that P 1 V 1 = P 2 V 2 61

62 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you squeeze a party balloon to 0.8 its volume, the pressure in the balloon __________. A.is 0.8 its former pressure B.remains the same if you squeeze it slowly C.is 1.25 times greater D.is 8 times greater Pressure in a Gas CHECK YOUR NEIGHBOR 62

63 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley When you squeeze a party balloon to 0.8 its volume, the pressure in the balloon __________. A. is 0.8 its former pressure B.remains the same if you squeeze it slowly C.is 1.25 times greater D.is 8 times greater Explanation: Boyles law, sweet and simple: P(1.0 V) = 1.25 P(0.8 V). Pressure in a Gas CHECK YOUR ANSWER 63

64 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Earths Atmosphere Atmosphere ocean of air exerts pressure The Magdeburg-hemispheres demonstration in 1654 by Otto von Guericke showed the large magnitude of atmospheres pressure. 64

65 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Atmospheric Pressure Atmospheric pressure caused by weight of air varies from one locality to another not uniform measurements are used to predict weather conditions 65

66 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Atmospheric Pressure Pressure exerted against bodies immersed in the atmosphere result from the weight of air pressing from above. at sea level is 101 kilopascals (101 kPa) weight of air pressing down on 1 m 2 at sea level ~ 100,000 N, so atmospheric pressure is ~ 10 5 N/m 2 66

67 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Atmospheric Pressure Pressure at the bottom of a column of air reaching to the top of the atmosphere is the same as the pressure at the bottom of a column of water 10.3 m high. Consequence: The highest the atmosphere can push water up into a vacuum pump is 10.3 m. Mechanical pumps that dont depend on atmospheric pressure dont have the 10.3-m limit. 67

68 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Atmospheric Pressure Barometer device to measure atmospheric pressure also determines elevation Aneroid barometer small portable instrument that measures atmospheric pressure calibrated for altitude, then an altimeter 68

69 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The maximum height to which water can be drunk through a straw __________. A.is 10.3 m B.is about 76 cm C.has no limit D.None of the above Atmospheric Pressure CHECK YOUR NEIGHBOR 69

70 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The maximum height to which water can be drunk through a straw __________. A.is 10.3 m B.is about 76 cm C.has no limit D.None of the above Explanation: However strong your lungs may be, or whatever device you use to make a vacuum in the straw, at sea level, the water could not be pushed up by the atmosphere higher than 10.3 m. Atmospheric Pressure CHECK YOUR ANSWER 70

71 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pascals Principle Pascals principle discovered by Blaise Pascal, a scientist and theologian in the 17 th century states that a change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid applies to all fluidsgases and liquids 71

72 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pascals Principle application in hydraulic press example: o pressure applied to the left piston is transmitted to the right piston o a 10-kg load on small piston (left) lifts a load of 500 kg on large piston (right) 72

73 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Pascals Principle application for gases and liquids o seen in everyday hydraulic devices used in construction o in auto lifts in service stations Increased air pressure produced by an air compressor is transmitted through the air to the surface of oil in an underground reservoir. The oil transmits the pressure to the piston, which lifts the auto. 73

74 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley In a hydraulic device, it is impossible for the __________. A.output piston to move farther than the input piston B.force output to exceed the force input C.output pistons speed to exceed the input pistons speed D.energy output to exceed energy input Pascals Principle CHECK YOUR NEIGHBOR 74

75 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley In a hydraulic device, it is impossible for the __________. A.output piston to move farther than the input piston B.force output to exceed the force input C.output pistons speed to exceed the input pistons speed D.energy output to exceed energy input Pascals Principle CHECK YOUR ANSWER 75

76 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in a Gas Archimedes principle applies to air as well as liquids. states that an object surrounded by air is buoyed up by a force equal to the weight of the air displaced 76

77 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A dirigible that hovers in air __________. A.displaces its volume of air B.displaces its weight of air C.Both A and B D.Neither A nor B Buoyancy in Gas CHECK YOUR NEIGHBOR 77

78 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley A dirigible that hovers in air __________. A.displaces its volume of air B.displaces its weight of air C.Both A and B D.Neither A nor B Explanation: Like a fish in water, both volume and weight of a fluid are displaced. Buoyancy in Gas CHECK YOUR ANSWER 78

79 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in Gas Rule for lighter-than-air objects: When the weight of air displaced by an object is greater than the weight of the object, it rises. When the weight of air displaced by an object equals the weight of the object, it hovers in air. When the weight of air displaced by an object is less than the weight of the object, it is not supported by air. 79

80 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in Gas Gas-filled balloons gas prevents atmosphere from collapsing them best buoyancy with hydrogen, the lightest gas (flammable, so seldom used) next-best buoyancy with helium heated air used in sports balloons As balloons rise, atmosphere becomes less dense with altitude. 80

81 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Buoyancy in Gas Gas-filled balloon will continue to rise until the weight of displaced air equals the total weight of the balloon the buoyant force on the balloon equals its weight (which says the same thing) 81

82 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley As a balloon rises higher and higher into the atmosphere, its __________. A.volume decreases B.density increases C.weight increases D.None of the above Buoyancy in Gas CHECK YOUR NEIGHBOR 82

83 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley As a balloon rises higher and higher into the atmosphere, its __________. A. volume decreases B.density increases C.weight increases D.None of the above Buoyancy in Gas CHECK YOUR ANSWER 83

84 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Fluid Flow continuous flow Volume of fluid that flows past any cross-section of a pipe in a given time is the same as that flowing past any other section of the pipe even if the pipe widens or narrows. Fluid speeds up when it flows from a wide to narrow pipe. Motion of fluid follows imaginary streamlines. 84

85 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Bernoullis Principle discovered by Daniel Bernoulli, a 15 th century Swiss scientist states that where the speed of a fluid increases, internal pressure in the fluid decreases applies to a smooth, steady flow 85

86 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Bernoullis Principle Streamlines thin lines representing fluid motion closer together, flow speed is greater and pressure within the fluid is less wider, flow speed is less and pressure within the fluid is greater 86

87 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Bernoullis Principle Laminar flow smooth steady flow of constant density fluid Turbulent flow flow speed above a critical point becomes chaotic 87

88 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The pressure in a stream of water is reduced as the stream speeds up. Can a stream of water from a fire hose actually knock a person off his or her feet? A.It cant, as Bernoullis principle illustrates. B.It can, and is consistent with Bernoullis principle. C.Bernoullis principle works only for laminar flow, which the stream is not. D.None of the above Bernoullis Principle CHECK YOUR NEIGHBOR 88

89 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley The pressure in a stream of water is reduced as the stream speeds up. Can a stream of water from a fire hose actually knock a person off his or her feet? A.It cant, as Bernoullis principle illustrates. B.It can, and is consistent with Bernoullis principle. C.Bernoullis principle works only for laminar flow, which the stream is not. D.None of the above Explanation: Theres a difference in the pressure within flowing water and the pressure it can exert when its momentum is changed. The pressure that knocks one off his or her feet is due to the change in the waters momentum, not the pressure within the water. Bernoullis Principle CHECK YOUR ANSWER 89

90 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Bernoullis Principle Applications of Bernoullis Principle Blow on the top surface of a paper and the paper rises. Reason: The pressure of the moving air is less than the atmospheric pressure beneath it. Convertible car roof puffs upward as air moves over its top surface. Reason: The pressure of air moving on the top surface is less than the atmospheric pressure inside the car. 90

91 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Bernoullis Principle Wind blowing across a peaked roof can lift the roof off the house. Reason: Pressure is reduced as wind gains speed as it flows over the roof. The greater pressure inside the house lifts the roof up. Perfume atomizer Reason: Air rushes across the open end of the tube when you squeeze the bulb, reducing pressure in the tube. Atmospheric pressure on the liquid below pushes it up into the tube. 91

92 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley Bernoullis Principle Trucks passing closely on a highway are drawn to each other. Reason: Air pressure between the trucks is less than pressure on their outer sides pushing inward. Risk of passing ships colliding sideways Reason: Water pressure between the ships is less than pressure on their outer sides pushing inward. 92

93 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley On a windy day, waves in a lake or the ocean are higher than their average height. What factor in Bernoullis principle contributes to the increased height? A.Reduced pressure pulls water upward. B.Pressure builds up over the crests. C.Air travels faster over the crests than the troughs. D.None of the above Bernoullis Principle CHECK YOUR NEIGHBOR 93

94 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley On a windy day, waves in a lake or the ocean are higher than their average height. What factor in Bernoullis principle contributes to the increased height? A.Reduced pressure pulls water upward. B.Pressure builds up over the crests. C.Air travels faster over the crests than the troughs. D.None of the above Explanation: The troughs of the waves are partially shielded from the wind, so air travels faster over the crests. Pressure there is thus lower than down below in the troughs. The greater pressure in the troughs pushes water into the even higher crests. Bernoullis Principle CHECK YOUR ANSWER 94


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